Add `:read_board` to GroupPolicy

What does this MR do?

Group boards can be read if you're allowed to read the group (:read_group permission). However projects use a permission, :read_board. Since the underlying functionality is mostly common, it helps having the same permission checks when possible.

The need for this arose from !22497 (merged), where we're adding querying boards using GraphQL. Having a common permission type to check for a board makes the definitions much cleaner, rather than having to check for :read_board for projects, and :read_group for groups.

So added the :read_board permission, which ends up being a sub permission of :read_group

Related to issue #36756 (closed)

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Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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